In a broad aspect, the present invention relates to methods and systems for monitoring and quantifying the movement of fluid in a target region. More particularly, the present invention is directed to methods and systems that allow for determination of the rate of fluid movement through a target region including those that comprise tissue or a vascular array.
Modern imaging techniques are rapidly improving the diagnostic accuracy and clinical management of pathological disorders. Among the more prevalent imaging technologies currently available are ultrasound, magnetic resonance imaging (MRI), computerized tomography (CT), and positron emission tomography (PET). Radiographic procedures, such as computed tomography and positron emission tomography, operate by detecting and mapping differences in the composition of a target object. Unfortunately CT and PET utilize ionizing radiation and require relatively expensive equipment. Conversely, MRI and ultrasound do not require ionizing radiation and, at least in the case of ultrasound imaging, utilize relatively inexpensive equipment.
In magnetic resonance visualization, advantage is taken of the fact that some atomic nuclei, such as, for example, hydrogen nuclei or fluorine nuclei have both nuclear spin and nuclear magnetic moment, and therefore can be manipulated by applied magnetic fields. Traditional MRI comprises the use of a magnetic field that is established across a body to align the spin axes of the nuclei of a particular chemical element in the direction of the magnetic field. The aligned, spinning nuclei execute precessional motions around the aligning direction of the magnetic field. For the aligned, spinning nuclei the frequency at which they precess around the direction of the magnetic field is a function of the particular nucleus which is involved and the magnetic field strength. The selectivity of this precessional frequency with respect to the strength of the applied magnetic field is very sharp and this precessional frequency is considered a resonant frequency.
After alignment or polarization of the selected nuclei, a burst of radio frequency energy at the resonant frequency is radiated at the target body to produce a coherent deflection of the nuclei spin alignment. When the deflecting radio energy is terminated, the deflected or disturbed spin axes are reoriented or realigned, and in this process radiate a characteristic radio frequency signal which can be detected by an external coil and then resolved by the MRI system to establish image contrast between different types of tissue in the body.
Contrast agents for MRI must possess a substantially different concentration of the nuclei used as a basis for scanning. For example, in a hydrogen scanning system, an agent substantially lacking hydrogen can be used. Conversely, in a magnetic visualization system that scans for a physiologically minor nucleus such as the fluorine nuclei, a substance with a high concentration of that nucleus would provide the appropriate contrast.
Imaging or contrast agents may be introduced into the body space in various ways depending on the imaging requirement. In the form of liquid suspensions or emulsions they may be placed into the area of interest by oral ingestion or injection into the bodily space (either directly or by channeling through selected vessels). Typically, the contrast agents are transported by the blood or other fluids to the regions of interest. A suitable contrast agent must be biocompatible, that is non-toxic and chemically stable, not absorbed by the body or reactive with the tissue, and eliminated from the body within a short time.
In ultrasound imaging, ultrasonic waves are transmitted into an object or patient via a transducer. As the sound waves propagate through the object or body, they are either reflected or absorbed by tissues and fluids. Reflected ultrasonic waves are then received by the transducer and converted into electrical signals from which an image is generated. The acoustic properties of the tissues and fluids determine the contrast which appears in the resulting image.
Ultrasound imaging, therefore, makes use of differences in tissue density and composition that affect the reflection of sound waves by those tissues. Images are especially sharp where there are distinct variations in tissue density or compressibility, such as at tissue interfaces. Interfaces between solid tissues, the skeletal system, and various organs and/or tumors are readily imaged with ultrasound.
Accordingly, in many imaging applications ultrasound performs suitably without use of contrast enhancement agents; however, for other applications, such as visualization of flowing blood in tissues, there have been ongoing efforts to develop agents to provide contrast enhancement. One particularly significant application for these contrast agents is in the area of vascular imaging. Such ultrasound contrast agents can improve imaging of flowing blood in the brain, heart, kidneys, lungs, and other tissues. This, in turn, facilitates research, diagnosis, surgery, and therapy related to the imaged tissues. A blood pool contrast agent also allows imaging on the basis of blood content (e.g., tumors and inflamed tissues) and can aid in the visualization of the placenta and fetus by enhancing only the maternal circulation.
In this regard, a variety of ultrasound contrast enhancement agents have been proposed. The most successful agents generally consist of microbubbles that can be injected intravenously. In their simplest embodiment, microbubbles are miniature bubbles containing a gas, such as air, and are formed through the use of foaming agents, surfactants or other film forming agents, or encapsulating agents. More advanced formulations of microbubbles, such as those described in U.S. Pat. No. 5,605,673, comprise fluorochemical gases or vapors. In any event, the microbubbles provide a physical object in the flowing blood that is of a different density and possesses a much higher compressibility than the surrounding fluid tissue and blood. As a result, these microbubbles act as good reflectors of ultrasound energy and can easily be imaged.
While contrast agents for both ultrasound and magnetic visualization can substantially enhance the resolution of physiological structures and highlight deficiencies in blood flow through tissue, their use has not, at least prior to the instant application, allowed for the reliable quantitative assessment of blood flow. For example, prior to the instant invention, common ways of determining the rate of blood flow included Doppler ultrasound or by introduction of a bolus of an imagable contrast agent into a coronary artery through a catheter and measurement of the transit time of the bolus through the heart. Unfortunately, Doppler measurements of vascular flow have not proven efficient enough to provide the necessary accuracy in clinical situations. Conversely, although more accurate, the introduction of a bolus of contrast agent into a coronary artery may lead to complications. Moreover, as the technique requires placement of a catheter directly into the coronary artery, it is extremely invasive. This substantially increases patient discomfort, burden on hospital resources and precludes the use of the procedure on all but the most serious cases. Perhaps most importantly, while the aforementioned techniques may be used to give a rough estimate of blood flow through the heart or in a major artery, current procedures are unable to accurately quantitate the rate of blood flow or perfusion within a particular tissue; i.e. within the liver, kidney or heart.
Accordingly, it is an object of the present invention to provide methods for the accurate determination for the rate or amount of blood flow through a particular target region as a factor of time.
It is another object of the present invention to provide for the accurate measurement of the perfusion rate of a selected target tissue.
It is still another object of the present invention to accurately provide for the noninvasive determination of blood flow through the heart.
It is yet another object of the present invention to provide for the accurate measurement of rate of fluid flow using ultrasound imaging or magnetic visualization techniques.
These and other objects are accomplished by the methods and systems of the present invention which, in a broad sense, provide for the accurate monitoring and/or measurement of fluid flow or perfusion in a selected structure. Preferably the structure is biological in nature and comprises a vascular or microvascular system. More preferably, the structure is a selected target region comprising tissue or at least a portion of an organ such as a kidney, liver, brain or heart. In such cases, the measured flow may comprise the flow of fluid through the structure (i.e. blood flow through the heart) or may comprise the rate of perfusion of a target region (i.e. permeation of an organ or tissue with blood). Moreover, the present invention provides for the determination of the selected values through relatively non-invasive means. Accordingly, the disclosed methods and systems may be used to safely monitor and/or diagnose physiological conditions.
Generally, the invention provides the desired data by observing the influx of imaging agent into the target region over a period of time. In preferred embodiments the present invention determines selected values, such as perfusion or flow rate (i.e. flow of fluid per unit volume per unit time), by eliminating or reducing a signal from an area of interest (target region) and subsequently detecting a return of the signal over a period of time. This general technique may also be used to provide the fluid exchange rate (i.e. fraction of fluid exchanged per unit time). The observed signal (or signal level) is associated with a contrast or imaging agent and increases in intensity as a function of agent concentration in a given target region. Preferably, the signal associated with the contrast or imaging agent may be reduced or eliminated by exposing the selected region to the appropriate form of energy including ultrasonic energy or magnetic pulses. Conversely, the signal may be enhanced or otherwise measurably altered through the application of energy or change in the applied magnetic field. Most commonly, agents compatible with the present invention will be associated with magnetic resonance visualization or ultrasound imaging although agents used in computerized tomography or positron emission tomography may also be compatible if they exhibit the necessary signal attenuation following exposure to the appropriate form of energy. In this respect, it will be appreciated that any contrast or imaging agent which exhibits a reduced signal following exposure to the selected form of energy is suitable for use with the present invention.
In any event, the present invention further provides a method for determining an exchange rate for a moving fluid in a target region comprising the steps of:
a. introducing undisrupted imaging agent into a moving fluid, said moving fluid being present in a target region;
b. allowing undisrupted imaging agent to penetrate said target region;
c. disrupting the imaging agent in the target region to provide disrupted imaging agent having a signal level less than that provided by undisrupted imaging agent;
d. allowing the moving fluid to transport undisrupted imaging agent from outside the target region into the target region;
e. interrogating the target region with a monitor capable of registering the signal level whereby an increase in signal level corresponding to an increase in undisrupted imaging agent is observed; and
f. calculating the exchange rate of moving fluid in the target region based on the observed increase in signal level.
It should be appreciated that, although the present invention will most often be described in terms of reducing the generated signal from the target region, it is not limited to such embodiments. That is, employment of ultrasound techniques will typically result in the disruption of the contrast agent and reduction of the signal in the target region. In contrast, magnetic resonance techniques may be used to enhance, reduce or otherwise measurably alter the amount and signature of the signal generated depending on the applied field and orientation of the agent nuclear spin. Accordingly, the present invention is not limited to the reduction and subsequent reappearance of generated signal in the target region but rather encompasses any measurable alteration of the signal in the target region and subsequent xe2x80x9crenormaliztionxe2x80x9d of the signal.
Another embodiment of the invention comprises a method for determining the rate of fluid exchange in a target region of a patient comprising:
a. introducing intact signal-generating contrast agent into a bodily fluid that moves into a target region such that the contrast agent is transported by said fluid into said target region;
b. rendering said contrast agent which is present in said target region at least partially disrupted;
c. thereafter allowing additional intact contrast agent to be carried into said target region by said bodily fluid; and
d. calculating the rate of fluid exchange in the target region based on the rate at which intact contrast agent enters the target region.
In particularly preferred embodiments, the signal generating moiety will comprise an ultrasound contrast agent such as a microbubble which is capable of being disrupted or destroyed by exposure to ultrasound energy. In other preferred embodiments, a magnetic resonance imaging agent may be employed. Using techniques well known to those skilled in the art, the selected imaging agent is introduced into the object or body to be imaged and allowed to permeate or perfuse the region of interest. Following introduction the region of interest, or target region, is exposed to ultrasonic, magnetic or other appropriate energy. The amount of power and length of exposure will depend on a number of factors but, when taken together, should be sufficient to reduce, enhance (i.e. in MRI) or otherwise measurably alter the signal received from the imaging agent in the target region upon subsequent interrogation. With regard to microbubbles, exposure to enough ultrasound energy will disrupt or destroy them, thereby reducing or eliminating the signal provided by the imaging agent. In any event, following disruption (or other alteration) of the generated signal, the target region will initially provide relatively little contrast, or measurably different contrast, when subsequently interrogated using conventional techniques.
After alteration or reduction of the signal in the target region, blood or other fluids gradually perfuse into, or flow through, the area of altered contrast bringing with them (i.e. transporting) undisrupted imaging agent. It is important to emphasize that the reduction of signal by exposure to ultrasound energy is a localized effect and that surrounding tissue and fluids still comprise sufficient levels of undisrupted contrast agent to essentially provide a full strength signal upon interrogation. Accordingly, as blood or fluids enter the target region, the localized concentration of contrast agent gradually increases thereby providing a stronger signal over time. Observing and measuring the relative increase in signal strength over selected periods allows for determination of the rate of fluid flow or perfusion and, consequently, quantification of the volumetric blood flow in a selected target region.
Observation of the increasing signal strength and determination of the desired values may be accomplished using conventional interrogation techniques such as ultrasound or magnetic resonance visualization. It will be appreciated that each interrogation of the target region may be used to produce a single snapshot or xe2x80x9cframexe2x80x9d of the area of interest. Preferably, the initial interrogation of the target region occurs concurrently with or following disruption of the imaging agent and prior to the introduction of any new, undisrupted imaging agent from the surrounding area. This original or xe2x80x9cbaselinexe2x80x9d frame records all the tissue and transducer artifacts and may be subtracted, digitally or otherwise, from subsequent interrogative frames (each produced by a discrete interrogation) to isolate changes in signal strength and increase the sensitivity of the measurements.
Those skilled in the art will appreciate that the interrogation of the target region may be performed at any desirable interval or intervals and may occur as often as necessary to produce an accurate measurement of fluid flow. In a preferred embodiment, localized disruption of the imaging agent is accomplished prior to each separate interrogation. That is, the imaging agent in the target region is disrupted by the application of power and, after a preselected interval, the area is interrogated using a compatible detection method. During, or immediately after, collection of the appropriate data, the imaging agent in the target region may be disrupted again to where the signal strength is approximately the same as it was in the baseline frame. In one embodiment, the image may be continuously disrupted with a pause of a selected interval prior to interrogation and return to the disrupted state. Alternatively, the target region is left undisturbed for a period sufficient to allow the local concentration of contrast agent and signal strength to return to partial or full predisruption levels. In either case the cycle or process (disruption and interrogation) may then be repeated using the same or different preselected interval subsequent to disruption and prior to interrogation. Within this embodiment it is immaterial that the imaging agent is disrupted during the interrogation step as the entire system to be imaged is to be xe2x80x9cresetxe2x80x9d i.e. by disrupting the imaging agent (and signal level) prior to any subsequent measurement. As previously alluded to, the interrogation step may actually comprise the disruption step (or the initial frame of the disruption step) in preferred embodiments. For such embodiments, the operator may advantageously use the same power settings for both the disruption and interrogation steps.
Accordingly, another embodiment of the invention comprises a method for determining the exchange rate of blood in a target region comprising the steps of:
a. introducing a microbubble contrast agent into moving blood of a mammal;
b. allowing the moving blood to transport said microbubble contrast agent to a preselected target region in said mammal;
c. applying localized ultrasonic power to said target region to disrupt at least a portion of the microbubble contrast agent therein;
d. interrogating the target region with an ultrasound scanner to provide a baseline frame representative of the localized signal level;
e. allowing the moving blood to transport undisrupted microbubble contrast agent into said target region whereby the signal level therein is increased;
f. subsequently interrogating the target region with an ultrasound scanner to provide at least one interrogative frame representative of the localized signal level; and
g. processing said baseline frame and said interrogative frame to calculate the rate of blood exchange based on the increase in target region signal level.
In another preferred embodiment, multiple interrogation steps may be performed sequentially to generate a plurality of interrogative frames without intervening disruption steps. However, when measurements are made using such procedures, it is preferable that the interrogations do not substantially disrupt the localized imaging agent or unduly reduce the signal strength. Accordingly, in such procedures the interrogation steps are typically carried out at lower power than the initial disruption step. While multiple interrogation methods may require intermittent adjustment of power levels and corresponding image reception scales, i.e. gray scale contrast in ultrasound, the procedure allows for more rapid acquisition of data and potentially more accurate measurements due to the number of sequential frames that may quickly be collected at varying intervals.
For example, when ultrasound imaging agents comprising microbubbles are used in the present invention an initial burst of approximately one second (at 60 frames per second) could be used to disrupt the bubbles in the target region. The last frame of this burst could be observed and used as a baseline frame where the amount of received signal is equated to zero. Note that this is a relative value and the absolute level of signal received is relatively immaterial for the purposes of the invention. The region of interest could then be interrogated using ultrasound after a short period, i.e. 0.2 seconds, with the received signal used to produce a discrete interrogation or interrogative frame. It will be appreciated that the primary detectable difference between the frame produced at 0.2 seconds and the baseline frame will be the signal generated by imaging agent that has flowed into the target region with blood or other fluids. Subtraction of the baseline image from the subsequent interrogative frame provides an easily quantifiable signal that is proportional to the amount imaging agent in the region of interest. This collected data may be used to calculate the desired values or could be combined with other derived measurements at the same or different preselected intervals to provide increasingly accurate flow or perfusion rates.
The selected values may be derived using as little as one interrogation frame. However, as described above, additional optional data may be collected by repeating the procedure with intervening localized disruption of the imaging agent or simply by sequentially producing multiple interrogation frames at substantially non-disrupting power. In the case of the former, the imaging agent in the target region would be disrupted to signal levels approximating that of the original baseline frame prior to subsequent interrogation. Of course a new baseline frame could be produced for each cycle and used in subsequent calculations with the respective interrogative frame. Disruption could occur during, or immediately following, the previous interrogation or after a period of time. Preferably the ultimate or maximum signal strength remains similar, i.e. within 10%, 20%, or 50% of the signal strength immediately prior to the initial disruption, during data collection. Following the second disruption, a second interrogation frame could be generated at the same interval (i.e. 0.2 seconds) or at any other interval (i.e. 0.5 seconds). Whatever interval is selected, the amount of signal strength may be determined by subtracting out the first or second baseline frame (which should be roughly equivalent if the same disruptive forces were used) from the second interrogation frame. That is, the original baseline frame could be subtracted from each interrogation frame despite the intervening disruption or, each baseline frame immediately proceeding the interrogative frame could be used to provide the relative signal level. In any case, this process or cycle of intervening disruption and interrogation may be performed as many times as desired while the imaging agent is present in sufficient quantity to produce a readable signal.
Alternatively, additional discrete interrogative frames could be produced in a like manner at selected intervals such as at 0.25 seconds, 0.5 seconds, 1.0 second, 2.0 seconds, 3.0 seconds, etc. after the first baseline frame. Of course, interrogation of the target region could also be conducted on an essentially continuous basis or with extremely short intervals (i.e. on the order of 100 frames per second) between signal level observation and the production of discrete frames. Essentially, the interrogation and signal measurement of the target region may be conducted at any desired interval or intervals. No intervening disruption of the imaging agent in the target region is undertaken. Accordingly, these interrogations preferably occur under conditions that do not substantially disrupt the imaging agent in the target region. For example, they may be conducted at lower power than the disruption event or for shorter intervals. As such, the strength of the received signals will be cumulative, i.e. the reading at 1.0 second will be greater than that received at 0.5 seconds due to the increase in imaging agent concentration during the intervening period.
Regardless of how the interrogative frames are generated, the target region is eventually interrogated at a point when the signal strength is approximately equivalent to what it was prior to the initial disruption of the agent, i.e. at 15 or 30 seconds to provide a xe2x80x9cmaximum signalxe2x80x9d frame. Those skilled in the art will appreciate that this maximum signal frame is not necessary to calculate the desired values (i.e. when using an exponential fit) and is therefore optional with regard to the invention. Of course, this maximum signal frame could also be derived prior to the initial disruption of imaging agent. In any case, knowing the relative signal strengths of the baseline frame (0%), maximum signal frame (100%) and intervening time point frames (interrogative frames), the rate of signal replacement (and hence the exchange rate and perfusion rate into the target region) may be easily determined using interpolation or other conventional mathematical techniques.
More particularly, the increase in signal per unit time divided by the strength of the signal in the maximum signal frame gives the volumetric fraction of a fluid (typically blood, plasma etc.) flowing through the tissue or organ (i.e. the fluid exchange rate). Knowing this rate, one can easily and accurately calculate the inverse of the average time it takes to replace all the fluid in the target region. This exchange rate, when multiplied by the fluid content of the tissue in the target region (easily derived from the literature or direct measurement) provides the perfusion rate (or flow rate) which is defined as milliliters of fluid flow per second per cubic centimeter of tissue. For the purposes of the present disclosure the term xe2x80x9cflow ratexe2x80x9d shall be held to be equivalent to the term xe2x80x9cperfusion ratexe2x80x9d and will be used interchangeably unless the context dictates otherwise. The fluid content of the tissue may further be derived by dividing the net signal level of the tissue by the signal level provided by a fast moving pure blood adjacent to the tissue i.e. a main vein or artery.
As previously indicated, it will be appreciated that any imaging or contrast agent capable of disruption by the application of energy is compatible with the present invention. Particularly preferred are ultrasound contrast agents comprising microbubbles or microballoons including those microbubbles comprising fluorinated gases or vapors. Such microbubbles may be, for example, in the form of free gas microbubbles, microbubbles encapsulated by relatively insoluble microspheres, microbubbles comprising a surfactant or tenside membrane, microbubbles formed by local supersaturation and microbubbles stabilized by liposomes or viscosity enhancing solutions. Similarly, imaging agents used for magnetic resonance visualization are also preferred for use in the present invention. Such imaging agents may typically be disrupted or inactivated by the application of bursts of magnetic energy which may alter the local environment of the imagable protons. Following disruption fluid flow may be measured by monitoring the reinfusion of active imaging agent using conventional magnetic visualization techniques.
Accordingly, in another aspect the present inventions comprises the use of fluorinated gases or vapors for the manufacture of a medicament. In particular, the invention comprises use of a fluorinated gas or vapor for the manufacture of a signal-generating contrast agent for determining the rate of fluid exchange in a target region of a patient whereby intact contrast agent is introduced into a flowing bodily fluid such that it is transported into the target region where the contrast agent is at least partially disrupted by the application of energy thereby allowing for calculation of the fluid exchange rate based on the observed rate at which intact contrast agent reinfuses the target region.
Using the aforementioned procedures and contrast agents the present invention allows determination of the rate and amount of blood flow in a target region or tissue, such as heart, brain, myocardium, liver, kidneys, lungs, and other tissue. Such information may be useful in locating and identifying stenotic arteries. Additionally, the present invention may be used to locate and identify dead or necrotic tissue, such as that resulting from heart attacks, strokes, or tissue rejection, since such necrotic tissue exhibits reduced blood flow rates.
Alternatively, the rate of blood flow may be used to locate and identify tissues having increased blood flow rates, such as certain regions of tumors or inflamed tissues. It will be appreciated therefore that the disclosed methods may be used for the detection, identification and classification of neoplasms. The present invention may also be used to measure the blood flow rate in the heart and its tissues during stress or exercise tests. Similarly, blood flow rates in myocardial tissue may be measured after oral or venous administration of drugs designed to increase the blood flow to a tissue. Also, changes in blood flow rates in myocardial tissue due to or during various interventions, such as coronary tissue vein grafting, coronary angioplasty, or use of thrombolytic agents (TPA or streptokinase) can also be measured. Blood flow rates may also be determined throughout the entire circulatory system, to assist in the diagnosis of general vascular pathologies and the viability of placental tissue.
The embodiments of the present method which utilize ultrasound as a detection technique include several beneficial features. For example, no knowledge of the transducer power is necessary to derive the perfusion rate. Furthermore, tissue attenuation and artifacts are eliminated from the signal. In addition, the rate of perfusion may be calculated without knowledge of the microbubble concentration, reflectivity, or power susceptibility. Finally, the present methods permit rapid and automatic calculation of the blood flow rate and tissue perfusion rate.
In addition to the aforementioned methods, the present invention further provides a systems for the determination of fluid exchange rates and perfusion rates. More specifically, one embodiment of the invention comprises a system for determining the rate of fluid exchange in a target region comprising:
a signal-generating ultrasound contrast agent;
an ultrasonic transducer capable of non-invasively disrupting at least a portion of the contrast agent present in a target region and observing a signal produced by the contrast agent upon insonation; and
a processor whereby said processor calculates the rate of fluid exchange in a target region based on transducer observed localized increases in contrast agent signal level following transducer mediated disruption of the contrast agent in the target region.
Of course, it will be appreciated that the disclosed systems are particularly useful for practicing the methods as described herein.
The present invention further provides devices which may be used to determine the fluid exchange rate and perfusion rate of selected target regions. In particular, the invention provides devices for non-invasively determining the rate of fluid exchange in a target region comprising:
an ultrasonic transducer capable of non-invasively disrupting at least a portion of an ultrasound contrast agent present in a target region and observing a signal produced by the contrast agent upon insonation to provide a plurality of ultrasound images;
a digital storage medium operably associated with said ultrasonic transducer wherein said digital storage medium receives data representative of said plurality of ultrasound images from the transducer; and
a processor operably associated with the digital storage medium whereby said processor calculates the rate of fluid exchange in the target region based on measurable differences in the ultrasound image data obtained from the digital storage medium.
With regard to the above-described devices, systems and methods it should be emphasized that the present invention is not limited to the determination of fluid flow rates in biological systems but may be applied to the determination of flow rates in non-biological systems as well. In addition, it should also be emphasized that imaging techniques other than ultrasound may also be utilized in conjunction with the present invention.
Other objects, features and advantages of the present invention will be apparent to those skilled in the art from a consideration of the following detailed description of preferred exemplary embodiments thereof.